Field of the Invention
This invention relates broadly to apparatus for processing variable repetitive input signals. More particularly, the invention relates to apparatus for selectively detecting variable repetitive input signals according to one of plural repetitive rates. The invention may be used with, among other things, moving bar or rod surface inspection equipment in a variablespeed rolling mill to facilitate identifying cracked rolls which produce repetitive transverse defects on the rolled product surface.
In such bar or rod rolling mills a roll crack may develop in any one of plural roll sets. Each set of rolls rotate at a different roll speed which is proportionate to one another. The resulting transverse defect is repeated lengthwise of the rolled product surface at a repetitive rate commensurate with cracked roll speed and location until the roll is changed. Consequently, the transverse defects may occur at any one of plural repetitive rates, depending on which of the plural roll sets contains the cracked roll.
In present highspeed bar or rod rolling mills, where bar or rod is rolled at respective speeds of up to 1220 mpm (about 4,000 fpm) and 3660 mpm (about 12,000 fpm), the rolled product is passed through surface inspection equipment to obtain full inspection of each rolled product before shearing or coiling operations. This procedure results in greater accuracy, reliability and less off-specification product than heretofore when only periodic sampling of the packaged order was made and this followed by a visual examination of the surface for cracked roll marks.
Prior art surface inspection equipment frequently comprises eddy-current apparatus having either a stationary or a rotating probe and an electronics package. The rolled product is passed through the probe and the electronics package generates a defect signal whenever the probe detects a defect on the surface of the rolled product. One of the problems with the prior art apparatus, both stationary and rotating probe type, is that they do not automatically detect repetitive defect signals caused by the cracked roll marks repeating lengthwise on the surface of the rolled product. Another problem with the prior art apparatus is that there is no correlation of repetitive defect signals with the particular roll set in a rolling mill having a cracked roll that causes the repetitive defect signals to be generated. In one instance, correlation circuitry is provided for distinguishing between true defect signals from false signals and noise, and to correlate defect angular position about a round bar surface but not a lengthwise correlation circuit. Even in data communication and acoustics arts where conventional auto- and cross-correlation techniques are used to enhance a signal over a noisy background, there is no teaching as to correlation of repetitive defect signals with a repetitive source causing the defect.